One form of high-performance burner is described in Markowski Ser. No. 397,395 and other improvements are described in the co-pending application of Lohmann et al, Ser. No. 968,652 having a common assignee with this application. In these constructions, the burner structure has axially spaced primary and secondary combustion zones which are structurally divided by a throat in the burner the purpose of which is to improve combustion in the secondary zone. Primary fuel is injected preferably in a wide spray to mix with and burn completely in the primary zone and secondary fuel is discharged in such a manner that it enters the throat before any significant secondary combustion occurs. Air and fuel mixing in the secondary zone is improved by short air swirler inlets generally in the form of pipes or tubes extending into the secondary zone and having swirlers therein to deliver a swirl of air into the secondary zone at a point spaced from the wall of the burner.
Although these air swirlers accomplish mixing to a great extent, adequate mixing requires several of these swirlers if the required low emissions are to be achieved. The throat also interferes with the desired flow pattern in the primary combustion zone and may affect the emissions at low power when the power plant is operating only on primary fuel. CO, NO.sub.x and unburned hydrocarbons are all serious problems at low power.